Fix typo in How to Contribute docs (#7614)
Fix typo in How to Contribute docs (#7614)

### Problem
There was a typo that resulted in a bad link.
![image](https://user-images.githubusercontent.com/8607745/56618902-80b89a80-6641-11e9-80ab-c344391f3e72.png)

### Solution
Use proper markdown syntax for links.

### Result
Now the actual URL won't be rendered, but instead the text will have the relevant hyperlink.
![image](https://user-images.githubusercontent.com/8607745/56618992-ba89a100-6641-11e9-9cb6-413d84bbaf3a.png)

pantsbuild/pants,wisechengyi/pants,wisechengyi/pants,tdyas/pants,twitter/pants,pantsbuild/pants,tdyas/pants,twitter/pants,twitter/pants,pantsbuild/pants,wisechengyi/pants,twitter/pants,jsirois/pants,pantsbuild/pants,tdyas/pants,pantsbuild/pants,tdyas/pants,benjyw/pants,benjyw/pants,benjyw/pants,twitter/pants,twitter/pants,tdyas/pants,tdyas/pants,pantsbuild/pants,jsirois/pants,wisechengyi/pants,tdyas/pants,twitter/pants,pantsbuild/pants,twitter/pants,jsirois/pants,wisechengyi/pants,wisechengyi/pants,wisechengyi/pants,benjyw/pants,tdyas/pants,benjyw/pants,tdyas/pants,wisechengyi/pants,wisechengyi/pants,benjyw/pants,benjyw/pants,twitter/pants